1. Field of the invention
The present invention relates to a friction coupling incorporating wear compensating means, such as a brake in particular, comprising friction means and means for compensating wear thereof. The invention is more particularly concerned with the structure of these wear compensating means.
2. Brief description of the prior art
A coupling of this kind, designed to couple by friction a support to which it is connected to an external member which moves relative to the support, comprises, for the purpose of applying its friction means against said external member, a clamping member coupled to a plunger adapted to move forwards or backwards when acted on by a control member. The wear compensation principle consists in maintaining at a substantially constant value the forward-return travel of this clamping member whilst progressively moving it closer to the external member as and when the friction means wear. In practice the friction means are placed on either side of this exteral member so as to clamp it between them.
The invention applies particularly, but not exclusively, to disk brakes for which safety requirements are extremely severe since, even where these brakes are intended for use as parking brakes, they may also serve as emergency brakes. This makes it necessary to provide for automatic compensation of the wear of the friction facings of these brakes, which is not the case with drum brakes, in which the brake shoes do not wear as quickly.
Friction couplings incorporating wear compensating means are generally of complex and delicate construction.
It is to alleviate this disadvantage that there is proposed in US patent application Ser. No. 624,906 filed June 26, 1984, now U.S. Pat. No. 4,598,801 a simple, convenient and reliable coupling which comprises an extendable plunger the extension of which is controled by a ratchet wheel the pawl of which is coupled to the control member; this wheel acts on the extendable plunger through the intermediary of a transmission member which is connected respectively by three calibrated clamping force friction couplings to the ratchet wheel, to the pawl and to a part of the plunger so that this transmission member is immobile relative to said plunger part when the pawl moves independently of the ratchet wheel but is driven when the pawl and the ratchet wheel move together. This transmission member is in practice a shaft disposed transversely relative to the plunger, which controls the relative unscrewing, within the plunger, of a screw which is fixed axially and a nut which is prevented from rotating; the calibrated clamping force friction couplings are acted on by an axially acting spring calibration member, the axial direction being parallel to the transmission shaft; the forward-return travel of the clamping member, mounted at the forward end of the plunger, oscillates between two values the difference between which is defined by the length of the teeth on the ratchet wheel. Wear compensation is thus discontinuous.
European patent application No. 0039479 described a wear compensating system in a brake in which extension of the plunger is controlled via two springs mounted coaxially with said plunger. However, the arrangement is such that the two springs operate simultaneously, with the intention of limiting the torque, so that any compensation of wear is effected on the forward braking stroke.
An objective of the present invention is a friction coupling incorporating wear compensating means which, whilst being simple, convenient and reliable, is adapted to provide continuous wear compensation during the return braking stroke, using the minimum number of components.